Elements Of Sensor Resolution

The smallest imaging element on an imaging sensor is the pixel.  Specifically, the light sensitive aperture of the pixel element.  It collects electrons that have been kicked into its potential well by incident photons during the exposure time. It then provides the digitizing component of the imaging system with a packet of charge.
In a non-color imaging system, the resolving element of the system is equal to the pixel (not considering any optical system degradation). Note that an electron kicked into the well by a 550 nm photon looks no different to the digitizer than an electron kicked into the well by a 450 nm photon.
In digital RGB color camera systems, the color of the light is determined by sampling light intensity in three bands of the visible spectrum: red (~530 nm – 700 nm), green (~460 nm – 600 nm) and blue (~400 nm – 500 nm). The color RGB digital image then typically consists of an array of data with three values for each pixel location corresponding to the red, green and blue color intensities for that location. Two popular methods in common use are Single Shot Color Mosaic Sampling and Three Shot Color Sampling: